Fragile X is the most common form of inherited mental retardation in males. It currently has no treatment. The pathology results from loss of expression of the Fmr1 gene. The protein product of Fmr1, FMRP, is an RNA binding protein that has been shown to interact with a limited subset of neuronal mRNAs. It is believed that translational regulation of these genes is the primary function of FMRP. We recently reported the presence of the Drosophila Fragile X homolog, dFxr, as a component of the RNA interference (RNAi) machinery. Based on this finding we developed the following hypothesis: FMRP functions as part of the RNAi machinery, and dysfunction of this shared biological machinery is responsible for Fragile X mental retardation. This hypothesis will be tested in this proposal. In the first Aim we will define the composition of this shared FMRP/RNAi machinery in mammalian cells. This will be done using fractionation methods and assays we have developed. In the second Aim we will identify mRNA targets that are regulated by this activity. In the third Aim we investigate the mechanism of gene regulation by the FMRP/RNAi machinery. This pathway is known to regulate mRNA translation. We have developed the hypothesis that translation is interrupted during the elongation step. We will employ polyribosome analysis of cell extracts and in vitro extracts to test the hypothesis. This work will have several important benefits. Functional validation of Fragile X targets will improve our understanding of Fragile X mental retardation, as well as providing new drug targets for the therapy of the human disease. This work will also define a role for RNAi in the regulation of mammalian genes, a biological pathway that is presently totally unknown. While this project focuses on FMRP regulation of neuronal genes, it will provide a general framework for studies on all aspects of RNAi regulation of mammalian cellular pathways.